Inkjet Ink And Printing Method Using The Same

ABSTRACT

Disclosed are an inkjet ink and a method for printing on hydrophobic media such as offset coated media using the inkjet ink. Specifically disclosed is an inkjet containing a pigment, water and a surfactant which is characterized by also containing 40 to 75 weight % of a polar non-protic solvent with a molecular weight of 40 to 130 which has permeability to a hydrophobic surface such as the coating of an offset coated medium. Such an inkjet ink enables better printing on hydrophobic media such as offset coated media than the conventional inks.

TECHNICAL FIELD

The present invention relates in particular to an inkjet ink suitablefor printing on hydrophobic media, such as offset coated media, and amethod for printing on hydrophobic media using the inkjet ink.

BACKGROUND ART

Generally with a conventional inkjet printing method using an aqueousinkjet ink, printing is performed on a bond paper or other normal typeof paper or on a specialized paper, typically designed to have a highink absorbing property, etc. However, offset coated media that aresuitable for offset printing are not suited for printing using anaqueous inkjet ink, because a substrate surface is coated with a smooth,hydrophobic, and non-porous coating.

Even if printing using an aqueous inkjet ink is performed, because thecoating does not absorb the aqueous inkjet ink, the inkjet ink isrepelled by or bleeds on the coating surface and a clear print withsharp edges cannot be obtained. Further, because the print is poor indrying property and inadequate in fixing property even upon drying,bleeding occurs readily when the print is rubbed. Development of a newart for performing printing satisfactorily on offset coated media by aninkjet printing method is thus being demanded.

For performing printing satisfactorily on offset coated media by aninkjet printing method, improvement of the affinity of an inkjet ink toa hydrophobic coating is considered first, and for this purpose,surfactant types have been examined and blending of glycol ether or awetting agent, etc., that exhibits compatibility with water and yetexhibits affinity with the hydrophobic coating has been proposed. Forimprovement of the fixing property as well as improvement of theaffinity of the printed inkjet ink on the coating, blending of awater-soluble or water-dispersible binder resin (vehicle) has also beenproposed (see, for example, Patent Documents 1 and 2, indicated below).

Combined use of water and a solvent, having compatibility with water andexhibiting permeability to a hydrophobic coating, is also considered,and the use of a polar aprotic solvent as such a solvent has beenproposed (Patent Document 3).

-   Patent Document 1: Japanese Unexamined Patent Publication No.    2003-206426 (Claims and Paragraphs 0009 to 0010)-   Patent Document 2: Japanese Unexamined Patent Publication No.    2004-510028 (Claims and Paragraph 0012)-   Patent Document 3: Japanese Unexampled Patent Publication No.    2003-268279 (Claims and Paragraph 0009)

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

With just examining surfactants, glycol ether, wetting agents, binderresins, etc., that are known components in aqueous inkjet ink's asdescribed in Patent Documents 1 and 2, the effects are limited and it isdifficult to perform better printing on a surface of an offset coatedmedium than at present.

On the other hand, since the polar aprotic solvent described in PatentDocument 3 exhibits good permeability to coatings that are normally usedin offset coated media, obtainment of clear prints with sharp edges byprevention of repulsion and bleeding of the inkjet ink on a coatingsurface, and improvement of the drying property of the printed inkjetink by absorption of the polar aprotic solvent in the coating, etc., arethus anticipated. However, with the arrangement of the inkjet inkdescribed in Patent Document 3, despite the inclusion of the polaraprotic solvent, the effects thereof are still inadequate and furtherimprovement is desired.

An object of the present invention is to provide an aqueous inkjet inkthat enables even better printing on hydrophobic media, such as offsetcoated media, than conventional inks, and a method for printing onhydrophobic media using the inkjet ink.

Means for Solving the Problem

The present invention relates to an inkjet ink for printing onhydrophobic media including: a pigment; water; a surfactant; and a polaraprotic solvent having a molecular weight of 40 to 130, wherein thecontent proportion of the polar aprotic solvent is 40 to 75 weight %.

The polar aprotic solvent preferably has a boiling point of 150 to 250°C. The polar aprotic solvent is preferably at least one type of solventselected from the group consisting of 1,3-dimethyl-2-imidazolidinone,N-methyl-2-pyrrolidone, 2-pyrrolidone, formamide, N-methylformamide,N,N-dimethylformamide, N,N-diethylpropionamide, and γ-butyrolactone.

As the surfactant, a surfactant having a dynamic contact angle at 23° C.of no more than 20° is preferable. Also, as the surfactant, asilicone-based surfactant is preferable, and a blending ratio (weightratio) P/S of the polar aprotic solvent P and the silicone-basedsurfactant S is preferably P/S=45/1 to 70/1.

An inkjet ink according to the present invention includes: a pigment;water; and polar aprotic solvent having a molecular weight of 40 to 130,wherein the content proportion of the polar aprotic solvent is 40 to 75weight %, asthe surfactant, a siiicone-based surfactant S₁ expressed bythe Formula (1):[Chemical Formula 1]

[where R¹ indicates a polyalkylene oxide chain containing at least oneor more ethylene oxide groups or propylene oxide groups, and x indicatesa numeral of no less than 1] and a silicone-based surfactant S₂expressed by the Formula (2):[Chemical Formula 2]

[where R² indicates a polyalkylene oxide chain containing at least oneor more ethylene oxide groups or propylene oxide groups, and y indicatesa numeral of no less than 1] are preferably used in combination, and ablending ratio (weight ratio) S₁/S₂ of the two silicone-basedsurfactants is S₁/S₂=6/4 to 4/6. Further, a blending ratio (weightratio) P/(S₁+S₂) of the polar aprotic solvent P and the twosilicone-based surfactants S₁+S₂ is preferably P/(S₁+S₂)=45/1 to 70/1.

An inkjet ink for printing on hydrophobic media comprising: a pigment;water; surfactant; and polar aprotic solvent having a molecular weightof 40 to 130, wherein the content proportion of the polar aproticsolvent is 40 to 75 weight %, and the surfactant is a fluorine-basedsurfactant expressed by the Formula (3):

[Chemical Formula 3]C_(m)F_(2m+1)COONH₄   (3)[where m indicates a numeral of no less than 1].

Also, a blending ratio (weight ratio) P/F of the polar aprotic solvent Pand the fluorine-based surfactant F is preferably P/F=45/1 to 100/1.Further, preferably, the pigment surface is modified by carboxylicgroups.

A printing method according to the present invention includes the stepsof: using the inkjet ink of the present invention to print on ahydrophobic medium; and heating the hydrophobic medium after printing.

EFFECTS OF THE INVENTION

In the inkjet ink described in Patent Document 3, an aprotic polarsolvent is contained at a content proportion of 5 to 40 weight % withrespect to the total amount of the inkjet ink. Examination by thepresent inventor has shown that within this content proportion range,the effect of increasing permeability into a coating or otherhydrophobic surface of an offset coated medium to improve the dryingproperty, etc., of a print cannot be adequately obtained.

Although in order to increase the permeability into a coating, etc., alarger amount of a polar aprotic solvent must be contained, when a polaraprotic solvent that is high in molecular weight is contained at aproportion of no less than 40 weight % with respect to the total amountof the inkjet ink, the viscosity of the inkjet ink increases and thismay cause degradation of discharge stability during discharge from finenozzles of a head of an inkjet printer.

In contrast, in the present invention, because among the polar aproticsolvents described in Patent Document 3, a polar aprotic solvent havinga low molecular weight of no more than 130 is used selectively and thecontent proportion thereof is limited to no less than 40 weight %, thepermeability with respect to a hydrophobic surface can be increased toimprove the drying property of a print, etc., over the conventional casewhile suppressing the increase of viscosity of the inkjet ink tomaintain good discharge stability. Thus, according to the inkjet ink ofthe present invention, better printing than at present can be performedon hydrophobic media such as offset coated media.

In the present invention, the molecular weight of the polar aproticsolvent is limited to no less than 40 because when a low molecularweight polar aprotic solvent with a molecular weight less than 40 isused, the inkjet ink dries readily and tends to readily cause cloggingof the above-described fine nozzles, etc. The content proportion of thepolar aprotic solvent is limited to no more than 75 weight % becausewhen the content proportion exceeds 75 weight %, even if the molecularweight of the polar aprotic solvent is limited to no more than 130, theviscosity of the inkjet ink becomes high and degrades the dischargestability during discharge from the fine nozzles.

In the present invention, the polar aprotic solvent preferably has aboiling point of 150 to 250° C. When the boiling point of the polaraprotic solvent is less than 150° C., the inkjet ink dries readily andtends to readily cause clogging of the nozzles, etc. When the boilingpoint exceeds 250° C., the inkjet ink conversely causes difficulty indrying and the drying property after printing may degrade. In contrast,when the boiling point of the polar aprotic solvent is 150 to 250° C.,the above problems can be prevented and even better printing can beperformed on hydrophobic media.

As a favorable polar aprotic solvent that meets these conditions, atleast one type of solvent selected from the group consisting of1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, 2-pyrrolidone,formamide, N-methylformamide, N,N-dimethylformamide,N,N-diethylpropionamide, and γ-butyrolactone can be cited. When any ofthese polar aprotic solvents is used, not only the various effectsmentioned above can be provided but, although the details are not clear,the dispersion stability of the pigment in the inkjet ink can also beimproved.

As the surfactant, a surfactant having a dynamic contact angle at 23° C.of no more than 20° is preferably used. When a surfactant having adynamic contact angle at 23° C. that exceeds 20° is used, the inkjet inkis heated even the inkjet ink does not wet hydrophobic surfacesadequately and thus a clear print with sharp edges may not be obtained.

Meanwhile, when a surfactant having a dynamic contact angle at 23° C. ofno more than 20° is used, because the inkjet ink wets hydrophobicsurfaces rapidly before heating, a clear print with sharp edges can beobtained.

As the surfactant having a dynamic contact angle at 23° C. of no morethan 20°, a silicone-based surfactant excellent in the effect oflowering the surface tension of the inkjet ink and improving the wettingproperty of the inkjet ink with respect to hydrophobic surfaces with nopossibility of environmental impact is used favorably in comparison toPFOS (perfluorooctane sulfonate) based surfactants excellent in the sameeffects.

The blending ratio (weight ratio) P/S of the polar aprotic solvent P andthe silicone-based surfactant S having a dynamic contact angle at 23° C.of no more than 20° is preferably P/S=45/1 to 70/1. When the amount ofthe silicone-based surfactant falls below this range, because thesilicone-based surfactant is excellent in compatibility with the polaraprotic solvent, the surfactant function of the silicone-basedsurfactant becomes obscured and the effect of improving the wettingproperty of the inkjet ink with respect to hydrophobic surfaces maybecome inadequate.

When the amount of the silicone-based surfactant exceeds the aboverange, the wetting property of the inkjet ink with respect tohydrophobic surfaces becomes too good, thereby causing the inkjet ink tobleed before heating, and a clear print with sharp edges may not beobtained. Further, because a part of the excessive silicone-basedsurfactant may become released and hydrolyzed to produce a siliconemonomer that is insoluble in water. This silicone monomer may undergophase separation from the inkjet ink and lower the uniformity of theinkjet ink, the discharge stability during discharge from theabove-described fine nozzles may degrade.

In contrast, when the blending ratio (weight ratio) P/S of the polaraprotic solvent P and the silicone-based surfactant S is set so thatP/S=45/1 to 70/1, the above problems can be prevented so as to enableeven better printing on hydrophobic media.

As the surfactant, a silicone-based surfactant S₁ expressed by the aboveFormula (1) and a silicone-based surfactant S₂ expressed by the Formula(2) are also preferably used in combination. When these two types ofsilicone-based surfactants are used in combination, good printing can beperformed with even higher stability on hydrophobic media of varioussurface states of a wide range, from glossy offset coated media with a60° glossiness, as measured, for example, by micro-Tri-gloss,manufactured by BYK-Gardner GmbH, of no less than 70°, to matted offsetcoated media, for which the above-described 60° glossiness is no morethan 20°. Moreover, as with the above-described surfactants, these twotypes of silicone-based surfactants provide a merit of no possibility toimpact the environment in comparison to the PFOS-based surfactants.

Although the silicone-based surfactant S₁ expressed by the Formula (1)exhibits an extremely high wetting property with respect to offsetcoated media and other hydrophobic media, so that good printing can beperformed on glossy offset coated media that readily repels aqueousinkjet inks. However, since the wetting property is too high, the printmay bleed on matted offset coated media due to the effects of the fineunevenness of the surface.

Meanwhile, using the silicone-based surfactant S₁ expressed by theFormula (1) in combination with the silicone-based surfactant S₂expressed by the Formula (2), which is more suppressed in wettingproperty than that of S₁, bleeding on the surfaces of matted offsetcoated media is prevented so that good printing can be performed on thematted offset coated media as well.

In the combined system, the blending ratio (weight ratio) S₁/S₂ of thetwo silicone-based surfactants is preferably set S₁/S₂=6/4 to 4/6. Whenthe amount of the silicone-based surfactant S₁ expressed by the Formula(1) falls below this range, because the effect of improving the wettingproperty of the inkjet ink by the silicone-based surfactant S₁ becomesinadequate, the inkjet ink may be readily repelled, especially inprinting on a glossy offset coated medium.

In contrast, when the amount of the silicone-based surfactant S₂expressed by the Formula (2) falls below the above range, because theeffect of suppressing the wetting property of the inkjet ink by thesilicone-based surfactant S₂ becomes inadequate, bleeding may occurreadily, especially in printing on a matted offset coated medium.

Meanwhile, when the blending ratio (weight ratio) S₁/S₂ of the twosilicone-based surfactants is set S₁/S₂=6/4 to 4/6, due to thesynergistic effect of combined use of the two silicone-basedsurfactants, printing can be performed with even higher stability onhydrophobic media of various surface states of a wide range, from glossyoffset coated media to matted offset coated media, as described above.

By the same reasons as those given above for the silicone-basedsurfactant having a dynamic contact angle at 23° C. of no more than 20°,the blending ratio (weight ratio) P/(S₁+S₂) of the polar aprotic solventP and the two silicone-based surfactants S₁ and S₂ is preferablyP/(S₁+S2)=45/1 to 70/1.

As the surfactant, a fluorine-based surfactant expressed by the Formula(3) can also be used preferably. When the fluorine-based surfactantexpressed by the Formula (3) is used, printing can be performed witheven higher stability on the above-described hydrophobic media ofvarious surface states of a wide range, from glossy offset coated mediawith which the above-described 60° glossiness is no less than 70° tomatted offset coated media with which the above-described 60° glossinessis no more than 20°. Moreover, as with the above-describedsilicone-based surfactants, the fluorine-based surfactant expressed bythe Formula (3) provides the merit that there is no possibility ofimpacting the environment in comparison to the PFOS-based surfactants.

A blending ratio (weight ratio) P/F of the polar aprotic solvent P andthe fluorine-based surfactant F is preferably set P/F=45/1 to 100/1.When the amount of the fluorine-based surfactant falls below this range,because the effect of improving the wetting property of the inkjet inkby the fluorine-based surfactant becomes inadequate, the inkjet ink maybe readily repelled, especially in printing on a glossy offset coatedmedium.

Further, when the amount of the fluorine-based surfactant is increasedin excess of the above range, not only no further effects is obtained,but also the inkjet ink entrains bubbles more readily, whereby thedischarge stability during discharge from the fine nozzles may degrade.Meanwhile, when the blending ratio (weight ratio) P/S of the polaraprotic solvent P and the fluorine-based surfactant F is set P/F=45/1 to100/1 these problems can be prevented and even better printing can beperformed on hydrophobic media.

The surface of the pigment contained in the inkjet ink is preferablymodified by carboxylic groups. The pigment can thereby be provided witha suitable hydrophilic property and be improved in dispersion stabilityin the inkjet ink.

In performing printing on an offset coated medium or other hydrophobicmedium using the above-described inkjet ink according to the presentinvention, drying by heating is preferably performed immediately afterprinting. The polar aprotic solvent can thereby be made to permeate intothe hydrophobic surface of the coating of the o 1set coated medium,etc., to aid the effect of improving the drying property of the inkjetink, whereby printing on a hydrophobic medium can be performed at aprinting speed substantially equivalent to printing using normal andaqueous inkjet ink on a surface of a water-absorbing printing object.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is now described.

<<Inkjet Ink>>

The present invention relates to an inkjet ink for printing onhydrophobic media including: a pigment, water, a surfactant, and a polaraprotic solvent having a molecular weight of 40 to 130, wherein thecontent proportion of the polar aprotic solvent is 40 to 75 weight %.

<Polar Aprotic Solvent>

As the polar aprotic solvent, a solvent having a molecular weight of 40to 130 is selected and used from among various polar aprotic solventsthat do not produce or receives hydrogen ions. The content proportion ofthis solvent is limited to 40 to 75 weight % with respect to the totalamount of the inkjet ink. The reasons for these are as have beendescribed above. Further, as described above, the polar aprotic solventhaving a boiling point of 150 to 250° C. is favorably used. Within theabove-described molecular weight range, the molecular weight of thepolar aprotic solvent is especially preferably 45 to 115 inconsideration of suppressing the increase of the viscosity of the inkjetink and further improving the discharge stability. Further, within theabove-described content range, the content proportion of the polaraprotic solvent is especially preferably 45 to 70 weight % inconsideration of suppressing the increase of the viscosity of the inkjetink and further improving the discharge stability.

As an example of a favorable polar aprotic solvent that meets the aboveconditions, at least one type of solvent selected from the groupconsisting of 1,3-dimethyl-2-imidazolidinone [molecular weight: 114.2,boiling point: 225.5° C.], N-methyl-2-pyrrolidone [molecular weight:99.1, boiling point: 202-C], 2-pyrrolidone [molecular weight: 85.1,boiling point: 245° C.], formamide [molecular weight: 45.0, boilingpoint: 210° C.], N-methylformamide [molecular weight: 59.1, boilingpoint: 197° C.], N,N-dimethylformamide [molecular weight: 73.1, boilingpoint: 153° C.], N,N-diethylpropionamide [molecular weight: 129.2,boiling point: 195° C.], and γ-butyrolactone [molecular weight: 86.1,boiling point: 204° C.] can be cited.

<Surfactant I>

Among the various conventionally known surfactants, a surfactant havinga dynamic contact angle at 23° C. of no more than 20° is preferable asthe surfactant. As a surfactant having a dynamic contact angle at 23° C.of no more than 20°, a silicone-based surfactant is preferable. Further,when the surfactant having a dynamic contact angle 23° C. of no morethan 20° is a silicone-based surfactant, the blending ratio (weightratio) P/S of the polar aprotic solvent P and the silicone-basedsurfactant S is preferably P/S=45/1 to 70/1 as described above. Thereasons for the above are as have been described already.

Although the lower limit of the dynamic contact angle is not limited inparticular, when the dynamic contact angle of the surfactant is too low,the wetting property of the inkjet ink with respect to a hydrophobicsurface becomes too good, thereby causing the inkjet ink to bleed beforeheating and a clear print with sharp edges may not be obtained. Thedynamic contact angle at 23° C. is preferably no less than 15°.

The dynamic contact angle is an angle formed by a droplet of a 0.1%aqueous solution of a surfactant to be measured and a substrate surfaceof a stainless steel SUS 304 as measured, for example, using anautomatic contact angle meter, such as CA-V, manufactured by KyowaInterface Science Co., Ltd., 300 ms after a contact of the droplet withthe substrate surface upon dropping the surfactant solution onto thesubstrate surface under an environment of 23° C.

Olfine (registered trade name) PD-501, manufactured by Nissin ChemicalIndustry Co., Ltd., can be cited as an example of a favorable surfactantthat meets the above conditions.

<Surfactant II>

In an inkjet ink according to another embodiment of the presentinvention, the combination of a silicone-based surfactant S₁ expressedby the Formula (1):[Chemical Formula 4]

[where R¹ indicates a polyalkylene oxide chain containing at least oneor more ethylene oxide groups or propylene oxide groups, and x indicatesa numeral of no less than 1] and a silicone-based surfactant S₂expressed by the Formula (2):[Chemical Formula 5]

[where R² indicates a polyalkylene oxide chain containing at least oneor more ethylene oxide groups or propylene oxide groups, and y indicatesa numeral of no less than 1] are used in combination as the surfactant.

In this combination, it is necessary that the blending ratio (weightratio) S₁/S₂ of the two silicone-based surfactants is S₁/S₂=6/4 to 4/6.Further, the blending ratio (weight ratio) P/(S₁+S₂) of the polaraprotic solvent P and the two silicone-based surfactants S₁ and S₂ ispreferably P/(S₁+S₂)=45/1 to 70/1. The reasons for these are as havebeen described above.

SILWET L-77, manufactured by Dow Corning Toray Silicone Co., Ltd.,KF-643, manufactured by Shin-Etsu Chemical Co., Ltd., etc., can be citedas favorable examples of the silicone-based surfactant expressed by theFormula (1). SILWET L-720, manufactured by Dow Corning Toray SiliconeCo., Ltd., can be cited as a favorable example of the silicone-basedsurfactant expressed by the Formula (2).

The pH of the inkjet ink using the above two types of silicone-basedsurfactants in combination is preferably adjusted to 7.3 to 7.7. Whenthe pH falls outside this range, the silicone-based surfactant expressedby the Formula (1) undergoes change with time and in particular, thewetting property of the inkjet ink with respect to a glossy offsetcoated medium may degrade. Further, if the pH is less than 7.3, when theinkjet ink is combined with a binder resin which is essentiallyinsoluble in water and is selectively soluble in an alkaline aqueoussolution having a basic substance dissolved therein to improve the waterresistance of the print, it may not be possible to dissolve the binderresin in the inkjet ink satisfactorily. A suitable basic substance canbe added to adjust the pH of the inkjet ink to fall within theabove-described range.

<Surfactant III>

In an inkjet ink according to still another embodiment of the presentinvention, a fluorine-based surfactant expressed by the Formula (3):

[Chemical Formula 6]C_(m)F_(2m+1)COONH₄   (3)[where m indicates a numeral of no less than 1] is used as thesurfactant. The reasons for this are as has been described already.

Surflon (registered trade name) S-111N, manufactured by Seimi ChemicalCo., Ltd., can be cited as a favorable example of the fluorine-basedsurfactant expressed by the Formula (3). The Surflon S-111N is asolution in which a mixture of a main component with m in the Formula(3) of 8 and slight amounts with m=10 or 12 are dissolved in a mixedsolvent of isopropyl alcohol and water, and the concentration of theabove compound as the effective component is 30 weight %.

<Pigment>

As the pigment, any inorganic and/or organic pigment that is normallyused in inkjet inks for inkjet can be used. As examples of inorganicpigments, one or more types among titanium oxide, iron oxide, and othermetal compounds, or carbon black, manufactured by a known method, suchas a contact method, furnace method, thermal method, etc., can be cited.

As examples of organic pigments, one or more types among azo pigments(including azo lakes, insoluble azo pigments, condensed azo pigments,and chelated azo pigments), polycyclic pigments (for example,phthalocyanine pigments, perylene pigments, perinone pigments,anthraquinone pigments, quinacridone pigments, dioxazine pigments,thioindigo pigments, isoindolinone pigments, quinophthalone pigments,etc.), dye chelates (for example, basic dye chelates, acidic dyechelates, etc.), nitro pigments, nitroso pigments, aniline black, etc.,can be cited.

Specific examples of yellow pigments, include C. I. Pigment Yellow 74,109, 110, and 138, specific examples of magenta pigments include C. I.Pigment Red 122, 202, and 209, specific examples of cyan pigmentsinclude C. I. Pigment Blue 15:3 and 60, specific examples of blackpigments include C. I. Pigment Black 7, specific examples of orangepigments include C. I. Pigment Orange 36 and 43, and specific examplesof green pigments include C. I. Pigment Green 7 and 36, etc.

One or more types of pigments may be used according to the color type ofthe inkjet ink. The content proportion of the pigment is preferably 0.1to 30 weight % with respect to the total amount of the inkjet ink.

The surface of the pigment is preferably modified and has hydrophilicgroups introduced thereon to provide the pigment with hydrophilicity andimprove its dispersion stability in the inkjet ink. Examples ofhydrophilic groups to be introduced on the surface of the pigment bymodification include carboxylic groups, sulfonic groups, etc. Carboxylicgroups are especially preferable. Whereas a pigment that is modified bysulfonic groups is too high in hydrophilicity and is such that thedispersion stability in the inkjet ink according to the presentinvention that contains a large amount of the polar aprotic solvent asdescribed above may become poor, a pigment that is provided withsuitable hydrophilicity by modification by carboxylic groups isexcellent in dispersion stability, especially in the inkjet inkaccording to the present invention. The pigment is preferably used inthe manufacture of the inkjet ink in a state of a pigment dispersion inwhich the pigment is dispersed in water using a dispersing agent.

In addition to the respective components described above, the inkjet inkaccording to the present invention may contain an ethylene oxide adductof an organic acid and other components to be described below.

<Ethylene Oxide Adduct of Organic Acid>

By making an ethylene oxide adduct of an organic acid salt contained inan inkjet ink, the discharge stability of the inkjet ink can beimproved, especially in a thermal jet (registered trade name) typeinkjet printer.

Although the reasons are not clear, it is considered that when in athermal jet type inkjet printer, an inkjet ink is heated instantaneouslyto a high temperature (approximately 400° C.), the above-describedethylene oxide adduct of an organic acid salt tends to become adsorbedonto the surface of the pigment so as to prevent the destruction of thedispersion stability of the pigment thereby to contribute to thedispersion stability.

Examples of the ethylene oxide adduct of the organic acid salt includecompounds, in which ethylene oxide is adducted to sodium salts,potassium salts, and other alkali metal salts or ammonium salts ofvarious mono- to tri-carboxylic acids, such as citric acid, gluconicacid, tartaric acid, lactic acid, D-malic acid, L-malic acid, etc.

In particular, in terms of improving the discharge stability of theinkjet ink, at least one type of compound, selected from the groupconsisting of ethylene oxide adducts of alkali metal salts of citricacid, such as sodium citrate and potassium citrate, and ethylene oxideadducts of ammonium citrate that are expressed by a Formula (4):[Chemical Formula 7]

[where M¹, M², and M³ may be the, same or may differ, with eachindicating sodium, potassium, or other alkali metal, the ammonium group,or hydrogen, but with not all of M¹, M², and M³ indicate hydrogen at thesame time, and n indicates a numeral from 1 to 28] is preferable.

A compound in the above Formula with which the n exceeds 28 may lowerthe discharge stability of the inkjet ink by increasing the viscosity ofthe inkjet ink or by separating in the inkjet ink due to reducingsolubility in water. The content proportion of the above compound ispreferably 0.1 to 5.0 weight % and more preferably 0.3 to 3.0 weight %with respect to the total amount of the inkjet ink. When the contentproportion is below this range, the above-described effects of improvingthe discharge stability of the inkjet ink provided by the containing ofthe compound may be inadequate. When the content proportion exceeds theabove range, clogging of the nozzles, etc., may occur.

<Polyoxyethylene Phenyl Ether and Polyoxyethylene Alkyl Phenyl Ether>

When the inkjet ink is made to contain at least one type of compoundselected from the group consisting of polyoxyethylene phenyl ethers,expressed by the Formula (5):[Chemical Formula 8]

[where p indicates a numeral from 3 to 28] and polyoxyethylene alkylphenyl ethers, expressed by the Formula (6):[Chemical Formula 9]

[where R³ indicates an alkyl group with 8 to 10 carbons and r indicatesa numeral from 3 to 28] the function of the above-described ethyleneoxide adduct of organic acid salt is aided and the discharge stabilityof the inkjet ink is thereby improved further.

With these compounds, the reason why the p in the Formula (5) expressingthe polyoxyethylene phenyl ether is preferably 3 to 28 is because acompound for which the p falls outside this range may be inadequate inthe effect of aiding the function of the ethylene oxide adduct oforganic acid salt. In particular, a compound with which the p in theabove Formula exceeds 28 may lower the discharge stability of the inkjetink by increasing the viscosity of the inkjet ink or by separating inthe inkjet ink due to reducing solubility in water.

As a specific example of the polyoxyethylene phenyl ether of the Formula(5), the compound expressed by the Formula (5-1):[Chemical Formula 10]

with which the p is 6, can be cited.

With the polyoxyethylene alkyl phenyl ether expressed by the Formula(6), the reason why the r in the Formula is preferably 3 to 28 and thenumber of carbons of the alkyl group indicated by R³ is preferably 8 to10 is because a compound for which the r falls outside the correspondingrange, or a compound for which the number of carbons of the alkyl groupindicated by R³ falls outside the corresponding range may be inadequatein the effect of aiding the function of the ethylene oxide adduct oforganic acid salt. In particular, a compound with which the r exceeds28, or a compound for which the number of carbons of the alkyl groupindicated by R³ exceeds 10 may lower the discharge stability of theinkjet ink by increasing the viscosity of the inkjet ink or byseparating in the inkjet ink due to reducing solubility in water.

As a specific example of the polyoxyethylene alkyl phenyl ether of theFormula (6), the compound expressed by a Formula (6-1):(Chemical Formula 11]

with which the r is 25 and the number of carbons of the alkyl groupindicated by R³ is 8 can be cited. Although compounds of Formula (6)include three types of compounds with which the alkyl group indicated byR³ is bound to the o-position, m-position, or p-position with respect tothe polyoxyethylene group on the phenyl group, any of these may be usedin the present invention. A mixture of two or more types of thesecompounds may be used as well.

The content proportion of the polyoxyethylene phenyl ether of theFormula (5) and/or the polyoxyethylene alkyl phenyl ether of the Formula(6) is preferably 0.1 to 7.0 weight % and more preferably 0.5 to 6.0weight % with respect to the total amount of the inkjet ink. When thecontent proportion is below this range, the above-described aidingeffect to aid stabilizing the discharge of the inkjet ink that isprovided by the containing of these compounds may be inadequate. Whenthe content proportion exceeds the above range, clogging may occurinside the head.

When a compound of one of either Formula (5) or (6) is used alone, theabove-described content proportion is the content proportion of thecompound on its alone, and when two or more types are used incombination, the above-described content proportion is the total contentproportion of the combined compounds. Because it is considered that thecompounds of the Formulae (5) and (6) differ in the mechanism of theaiding effect, combined use of both is preferable. A combined system ofthe compound of Formula (5-1) and the compound of Formula (6-1) isespecially preferable in terms of the aiding effect.

<Acetylene Glycols and Glycol Ethers>

When an acetylene glycol and/or a glycol ether is contained in theinkjet ink, because these compounds provide a supplementing effect tosupplement the aiding effect of the polyoxyethylene phenyl ether of theFormula (5) and/or the polyoxyethylene alkyl phenyl ether of the Formula(6), the discharge stability of the inkjet ink can be further improved.

Among these, as examples of acetylene glycols, one or more types ofcompounds among Surfynol (registered trade name) 104 and other productsof this series manufactured by Air Products and Chemicals Inc., Surfynol420, 440, 465, and 485, manufactured by the same, Dynol 604,manufactured by the same, Olfine E4001, 4036, and 4051, manufactured byNissin Chemical Industry Co., Ltd., etc., can be cited.

The content proportion of the acetylene glycol is preferably 0.01 to 5.0weight % and more preferably 0.05 to 3.0 weight % with respect to thetotal amount of the inkjet ink. When the content proportion is belowthis range, the above-described supplementing effect that is provided bycontaining the compound may be inadequate. When the content proportionexceeds the above range, the water resistance of the print may degrade.

When an acetylene glycol expressed by the Formula (7):[Chemical Formula 12]

[where each of s and t separately indicates a numeral from 0 to 40, buts and t are not both 0 at the same time and s+t indicates numeral from 1to 40] is combined with the above-described combined system of the twotypes of silicone-based surfactant or with the fluorine-based surfactantof the Formula (3), an effect of improving the wetting property of theinkjet ink, especially with respect to glossy offset coated media can beprovided in addition to the above-described supplementing effect.

As examples of acetylene glycols expressed by the Formula (7), Surfynol420 [made up of a mixture of a plurality of components of the Formula(7) in which the values of s and t differ, with the average value of s+tbeing 1.3], Surfynol 440 [made up of a mixture of a plurality ofcomponents of the Formula (7) in which the values of s and t differ,with the average value of s+t being 3.5], Surfynol 465 [made up of amixture of a plurality of components of the Formula (7) in which thevalues of s and t differ, with the average value of s+t being 10], andSurfynol 485 [made up of a mixture of a plurality of components of theFormula (7) in which the values of s and t differ, with the averagevalue of s+t being 30], respectively manufactured by Air Products andChemicals Inc., can be cited from among the various compounds given asexamples above.

Meanwhile, as examples of glycol ethers, one or more types of compoundsamong ethylene glycol monobutyl ether, diethylene glycol monobutylether, triethylene glycol monobutyl ether, ethylene glycol monoisobutylether, diethylene glycol monoisobutyl ether, ethylene glycol monohexylether, diethylene glycol monohexyl ether, ethylene glycolmono-2-ethylhexyl ether, diethylene glycol mono-2-ethylhexyl ether,etc., can be cited.

The content proportion of the glycol ether is preferably 0.5 to 10.0weight % and more preferably 2.0 to 7.0 weight % with respect to thetotal amount of the inkjet ink. When the content proportion is belowthis range, the above-described supplementing effect that is provided bycontaining the compound may be inadequate. When the content proportionexceeds the above range, because the compound is a non-volatile liquid,it may become difficult to dry the inkjet ink. The storage stability ofthe inkjet ink may also degrade.

<Binder Resin I>

When a binder resin is contained in the inkjet ink, because the binderresin functions as a binder of the pigment and the hydrophobic surface,such as coating of an offset coated medium, the water resistance and theabrasion resistance of the print, the sharpness of the print, etc., canbe improved. In particular, when a binder resin that is essentiallyinsoluble in water and is selectively soluble in an alkali aqueoussolution in which a basic compound is dissolved, is used as a binder,the water resistance of the print can be further improved.

As an alkali-soluble binder resin, for example, a resin is preferablyhaving carboxylic groups in the molecule and insoluble in water as itis, while the resin is added in an alkali aqueous solution containing abasic substance, such as ammonia, organic amine, caustic alkali, etc.,the carboxylic group portion thereof reacts with the basic substance andthe resin dissolves in the solution with producing a water soluble salt.

As favorable examples, one or more types of resins, especially, highacid value resins, among acrylic resins, such as polyacrylic acid,acrylic acid-acrylonitrile copolymers, potassium acrylate-acrylonitrilecopolymers, vinyl acetate-acrylate copolymers, and acrylic acid-alkylacrylate copolymers; styrene-acrylic acid resins, such asstyrene-acrylic acid copolymers, styrene-methacrylic acid copolymers,styrene-methacrylic acid-alkyl acrylate copolymers, styrene-α-methylstyrene-acrylic acid copolymers, and styrene-α-methyl styrene-acrylicacid-alkyl acrylate copolymers; and maleic acid resins, fumaric acidresins, styrene-maleic acid copolymer resins, styrene-maleic anhydridecopolymer resins, etc., that are adjusted in molecular weight, acidvalue, etc., so as to have the characteristics described above, can becited.

Among the above, an alkali-soluble acrylic resin is especiallypreferable, and as specific examples, NeoCryl (registered trade name)B-817 (weight average molecular weight Mw: 23,000) and NeoCryl B-890(weight average molecular weight Mw: 12,500), manufactured by AveciaLimited, and Johncryl (registered trade name) 67 (weight averagemolecular weight Mw: 12,500), manufactured by Johnson PolymerCorporation, can be cited.

In consideration of improving the water resistance and the abrasionresistance of the print, the alkali-soluble binder resin preferably hasa weight average molecular weight Mw of no less than 10,000. However,when the molecular weight is too high, deposition, precipitation, etc.,of the binder resin occur readily and the discharge of the inkjet inkmay become unstable. Deposition, precipitation, etc., of the binderresin during storage of the inkjet ink may also occur readily. Theweight average molecular weight Mw of the binder resin is preferably nomore than 50,000 within the above range, and in consideration of thesefactors, is more preferably approximately 20,000 to 40,000.

The content proportion of the binder resin is preferably 0.1 to 3.0weight % and more preferably 0.5 to 2.0 weight % with reespect to thetotal amount of the inkjet ink. When the content proportion is belowthis range, the effect of fixing the pigment onto the non-aqueoussurface, etc., and improving the water resistance and the abrasionresistance of the print and the sharpness of the print may becomeinadequate. In the case where the content proportion exceeds the aboverange, when the viscosity of the inkjet ink inside a printer headincreases locally in accordance with the vaporization of water in athermal jet system as mentioned above, the discharge of the ink maybecome unstable and excess binder resin may separate to cause clogginginside the head.

<Binder Resin II>

An inkjet printer normally has a mechanism for capping and closingnozzles when a head is returned to a home position during a state ofnon-use. However, there are arrangements in which the operation ofreturning the head to the home position is performed manually, and withsuch an inkjet printer, when the head return to the home position isforgotten and the nozzles are left in an uncapped state for a while, theviscosity of the inkjet ink increases to cause faints and other printdefects readily in initial stages when printing is restarted.

Thus, in the inkjet ink for an inkjet printer of the type in which theoperation of returning the head to the home position is performedmanually, the use of a binder resin with a weight average molecularweight Mw of 1,000 to 3,000 and especially 1,000 to 2,000 is preferablefor preventing faints and other print defects reliably. Because aninkjet ink that contains a binder resin of such low weight averagemolecular weight Mw does not increase viscosity suddenly even if thenozzles are left uncapped, faints and other print defects are notproduced in the initial stages when printing is restarted.

When the binder resin with the weight average molecular weight Mw of1,000 to 3,000 is used in an aqueous inkjet ink and printing isperformed on general paper, etc., the effect of improving the waterresistance and the abrasion resistance cannot be obtained. However, withthe inkjet ink according to the present invention which contains thepolar aprotic solvent at the large amount of 40 to 75 weight % andprints by permeation into the coating of an offset coated medium whileslightly dissolving the coating by the polar aprotic solvent, even ifthe binder resin with the weight average molecular weight Mw of 1,000 to3,000 is used, the effect of improving the water resistance and theabrasion resistance can be improved adequately to levels without problemin terms of practical use.

The other characteristics of the binder resin are preferably the same asthose of binder resins of high molecular weight described above. Thatis, preferably in terms of improving the water resistance of the print,the binder resin is essentially insoluble in water and selectivelysoluble in an alkali aqueous solution in which a basic substance isdissolved. As an example of such a binder resin with a weight averagemolecular weight Mw of 1,000 to 3,000 that satisfies these conditions,Johncryl 682 (weight average molecular weight Mw: 1,700), manufacturedby Johnson Polymer Corporation, can be cited.

The content proportion of the binder resin is preferably 0.1 to 3.0weight % and more preferably 0.5 to 2.0 weight % with respect to thetotal amount of the inkjet ink. When the content proportion is belowthis range, the print may not be provided with water resistance andabrasion resistance of levels without problem in terms of practical useand sharp printing may not be possible. When the content proportionexceeds the above range, faints may occur in the print and nozzles maybecome clogged.

<Basic Substance>

The basic substance is used to make the inkjet ink alkaline to dissolvethe binder resin as mentioned above, to prevent corrosion of the head,and to maintain the dispersion stability of the pigment. As the basicsubstance, ammonia, an organic amine, or a caustic alkali, etc., ispreferable as mentioned above.

As examples of the organic amine, one type or two or more types ofcompounds among monoethanolamine, diethanolamine, triethanolamine,ethylmonoethanolamine, ethyldiethanolamine, monoisopropanolamine,diisopropanolamine, trilsopropanolamine, mono-1-propanolamine,2-amino-2-methyl-1-propanol, and derivatives thereof can be cited.

The added amount of the basic substance can be adjusted appropriatelyaccording to the acid value that indicates the free fatty acid contentof the binder resin or the pH of the inkjet ink in the state in whichthe basic substance is not added. In general, the added amount ispreferably 0.05 to 2 parts by weight and more preferably 0.075 to 1.5parts by weight to 1 part by weight of the binder resin. When the addedamount is below this range, because the binder resin cannot be dissolvedadequately, a uniform inkjet ink may not be obtained. When the contentexceeds the above range, the pH of the ink becomes too high and maycause problems in terms of safety or may cause corrosion of the head ofthe inkjet printer.

<Other Additives>

In addition to the respective components described above, variousconventionally known additives for inks may be added to the inkjet inkcomposition according to the present invention. Such additives include,for example, pigment dispersing agents, antifungal agents, biocidalagents, etc.

Besides the use in the above-described thermal jet type, piezo type, andother so-called on-demand type inkjet printers, the manufactured inkcomposition can be used in so-called continuous type inkjet printers inwhich printing is performed by forming ink droplets while circulatingthe ink.

<<Printing Method>>

The printing method according to the present invention is characterizedin including the steps of printing on a hydrophobic medium by an inkjetprinting method using the inkjet ink according to the present inventionand heating the hydrophobic medium after printing.

In the printing step, printing is performed under normal conditionsusing an above-described thermal jet type, piezo type, or otherso-called on-demand type inkjet printer or a continuous type inkjetprinter, etc. In the heating step, a heater is positioned incontinuation to a printed object exit of the inkjet printer and heatingat a predetermined temperature can be performed for a predeterminedtime. Although the conditions of heating temperature, heating time,etc., are not limited in particular, so as not to extend the timerequired for printing, it is preferable to set the heating time to nomore than 10 seconds and preferably to approximately 1 to 5 seconds.

EXAMPLES

The present invention shall now be described based on Examples andComparative Examples.

Example 1

<Manufacture of the Inkjet Ink>

A carbon black dispersion [Cabojet 300, manufactured by CabotCorporation, carboxylic group modified, aqueous dispersion, 15 weight %of solids] was used as the pigment, and 1,3-dimethyl-2-imidazolidinone[molecular weight: 114.2, boiling point: 225.5° C.] was used as thepolar aprotic solvent. A silicone-based surfactant [Olfine PD-501,manufactured by Nissin Chemical. Industry Co., Ltd., dynamic contactangle at 23° C.: 16.1°] was used as the surfactant, and an ethyleneoxide adduct of sodium citrate with which the p in the Formula (4) is 6and all of M¹ to M³ are sodium was used as the ethylene oxide adduct oforganic acid.

As the binder resin, an alkali-soluble acrylic resin [NeoCryl B-817,manufactured by Avecia Limited, weight average molecular weight Mw:23,000] was used, and as the basic substance for dissolving the binderresin in the inkjet ink, 2-amino-2-methyl-1-propanol was used. As thepolyoxyethylene phenyl ether, the compound expressed by the Formula(5-1) was used, and as the polyoxyethylene alkyl phenyl ether, thecompound expressed by the Formula (6-1) was used. As the acetyleneglycols, Olfine E4001, manufactured by Nissin Chemical Industry Co.,Ltd., was used, and as the biocidal agent, Proxel (registered tradename) XL-2, manufactured by Zeneca Co., Ltd. was used.

The respective components mentioned above were blended at theproportions given below in ion-exchanged water, and after mixing bystirring, the mixture was filtered using a membrane filter of 5 μm tomanufacture the inkjet ink. The blending ratio (weight ratio) P/S of thepolar aprotic solvent P and the silicone-based surfactant S was 61/1.(Component) (Parts by weight) Carbon black dispersion 151,3-dimethyl-2-imidazolidinone 48.5 Silicone-based surfactant 0.8 Binderresin 1 2-amino-2-methy-1-propanol 0.2 Ethylene oxide adduct of sodiumcitrate 0.5 C₆H₅O(CH₂CH₂O)₆H 0.5 C₈H₁₇C₆H₄O(CH₂CH₂O)₂₅H 0.5 Ethanol 2Acetylene glycol 1 Biocidal agent 0.2 Ion-exchanged water 29.8

Examples 2 to 8

In place of 1,3-dimethyl-2-imidazolidinone, except using the sameamounts of N-methyl-2-pyrrolidone [molecular weight: 99.1, boilingpoint: 202° C.] (Example 2), 2-pyrrolidone [molecular weight: 85.1,boiling point: 245° C.] (Example 3), formamide [molecular weight: 45.0,boiling point: 210° C.] (Example 4), N-methylformamide [molecularweight: 59.1, boiling point: 197° C.] (Example 5), N,N-dimethylformamide[molecular weight: 73.1, boiling point: 153° C.] (Example 6),N,N-diethylpropionamide [molecular weight: 129.2, boiling point: 195°C.] (Example 7), and γ-butyrolactone [molecular weight: 86.1, boilingpoint: 204° C.] (Example 8) as the polar aprotic solvents, inkjet inkswere manufactured in the same manner as in Example 1.

Example 9

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar aprotic solvent to 40 parts by weight, the amount of thesilicone-based surfactant to 0.7 parts by weight, the ratio P/S of thetwo components to 57/1, and the amount of the ion-exchanged water to38.4 parts by weight, an inkjet ink was manufactured in the same manneras in Example 1.

Example 10

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar aprotic solvent to 70 parts by weight, the amount of thesilicone-based surfactant to 1.1 parts by weight, the ratio P/S of thetwo components to 64/1, and the amount of the ion-exchanged water to 8parts by weight, an inkjet ink was manufactured in the same manner as inExample 1.

Example 11

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar aprotic solvent to 75 parts by weight, the amount of thesilicone-based surfactant to 1.2 parts by weight, and the ratio P/S ofthe two components to 63/1, and the amount of the ion-exchanged water to2.9 parts by weight, an inkjet ink was manufactured in the same manneras in Example 1.

Example 12

In place of Olfine PD-501, except using the same amount of asilicone-based surfactant with which the dynamic contact angle at 23° C.is 19.80 as the silicone-based surfactant, an inkjet ink wasmanufactured in the same manner as in Example 1.

Example 13

In place of Olfine PD-501, except using the same amount of asilicone-based surfactant with which the dynamic contact angle at 23° C.is 24.30 as the silicone-based surfactant, an inkjet ink wasmanufactured in the same manner as in Example 1.

Example 14

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar aprotic solvent to 48 parts by weight, the amount of thesilicone-based surfactant to 1.2 parts by weight, the ratio P/S of thetwo components to 40/1, and the amount of the ion-exchanged water to29.9 parts by weight, an inkjet ink was manufactured in the same manneras in Example 1.

Example 15

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar aprotic solvent to 49.5 parts by weight, the amount of thesilicone-based surfactant to 1.1 parts by weight, the ratio P/S of thetwo components to 45/1, and the amount of the ion-exchanged water to18.5 parts by weight, an inkjet ink was manufactured in the same manneras in Example 1.

Example 16

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar solvent to 48.5 parts by weight, the amount of the silicone-basedsurfactant to 0.7 parts by weight, the ratio P/S of the two componentsto 69/1, and the amount of the ion-exchanged water to 29.9 parts byweight, an inkjet ink was manufactured in the same manner as in Example1.

Example 17

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar aprotic solvent to 52.5 parts by weight, the amount of thesilicone-based surfactant to 0.7 parts by weight, the ratio P/S of thetwo components to 75/1, and the amount of the ion-exchanged water to25.9 parts by weight, an inkjet ink was manufactured in the same manneras in Example 1.

Example 18

In place of Cabojet 300, except using the same amount of a carbon blackdispersion [Cabojet 200, manufactured by Cabot Corporation, sulfonicgroup modified, aqueous dispersion, 15 weight % of solids] as thepigment, an inkjet ink was manufactured in the same manner as in Example1.

Comparative Example 1

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar aprotic solvent to 35 parts by weight, the amount of thesilicone-based surfactant to 0.6 parts by weight, the ratio P/S of thetwo components to 61/1, and the amount of the ion-exchanged water to33.5 parts by weight, an inkjet ink was manufactured in the same manneras in Example 1.

Comparative Example 2

Except setting the amount of the 1,3-dimethyl-2-imidazolidinone as thepolar aprotic solvent to 77.9 parts by weight, the amount of thesilicone-based surfactant to 1.2 parts by weight, the ratio P/S of thetwo components to 66/1, and not blending in the ion-exchanged water, aninkjet ink was manufactured in the same manner as in Example 1.

Comparative Example 3

Except using N,N-dibutylformamide [molecular weight:

157.3, boiling point: 240° C.] as the polar aprotic solvent, an inkjetink was manufactured in the same manner as in Example 1.

The following tests were carried out on the inkjet inks manufactured inthe respective Examples and Comparative Examples to evaluate thecharacteristics of the inks.

<<Discharge Stability Test I>>

Each of the inkjet inks of the Examples and Comparative Examples wasfilled into an empty black ink cartridge [Part No. 51645A, manufacturedby Hewlett-Packard Development Company, L.P.] for a thermal jet typeinkjet ink printer [Desk Jet 970Cxi, manufactured by Hewlett-PackardDevelopment Company, L.P.], and a line of a line width of 0.5 points wasprinted on a commercially available glossy paper. The print was thenobserved, and the discharge stability of the inkjet ink was evaluatedaccording to the following standards.

Excellent: No faints were seen at all at the start of printing, and theline was printed without interruption in the middle. The dischargestability was evaluated extremely good.

Good: Although faints were seen at the start of printing, thereafter,the line was printed without interruption in the middle. The dischargestability was evaluated as reaching a practical level.

Poor: The line was interrupted in the middle. The discharge stabilitywas evaluated poor.

<<Drying Property Test>>

Using the same thermal jet type inkjet ink printer [Desk Jet 970Cxi,manufactured by Hewlett-Packard Development Company, L.P.] as describedabove, printing by each of the inkjet inks of the Examples andComparative Examples on an offset coated medium (packaging material ofthe above-described ink cartridge) was performed, and then after heatingfor 2 seconds using an oven toaster at an output of 500 W, the print wasrubbed with a cotton swab while applying a load of 20 g. The print wasthen observed, and the drying property of the inkjet ink was evaluatedaccording to the standards below. Measurement of glossiness by amicro-Tri-gloss manufactured by BYK-Gardner GmbH showed 60° glossinessof the offset coated medium was 46.5°.

Excellent: No bleeding was seen at all. The drying property wasevaluated extremely good.

Good: Although slight bleeding was seen, the drying property wasevaluated as reaching a practical level.

Poor: Major bleeding was seen, and the drying property was evaluatedpoor.

<<Print Sharpness Test I>>

Using the same thermal jet type inkjet ink printer [Desk Jet 970Cxi,manufactured by Hewlett-Packard Development Company, L.P.] as describedabove, printing alphabetical characters of 10-point by each of theinkjet inks of the Examples and Comparative Examples on an offset coatedmedium (packaging material of the above-described ink cartridge) wasperformed, and then after heating for 2 seconds using an oven toaster atan output of 500 W, the print was observed and the print sharpness ofthe inkjet ink was evaluated according to the following standards.

Excellent: Edges were expressed sharply. The print sharpness wasevaluated extremely good.

Good: Although there were some portions that somewhat lack sharpness,the print sharpness was evaluated as reaching a practical level.

Poor: The print was not sharp, and the print sharpness-was evaluatedpoor.

The results of the above are shown in Tables 1 to 3. TABLE 1 Example 1 23 4 5 6 7 Polar aprotic Molecular weight 114.2 99.1 85.1 45.0 59.1 73.1129.2 solvent P Content 48.5 48.5 48.5 48.5 48.5 48.5 48.5 proportion(wt %) Dynamic contact angle of the 16.1° 16.1° 16.1° 16.1° 16.1° 16.1°16.1° surfactant S P/S 61/1 61/1 61/1 61/1 61/1 61/1 61/1 Modificationof the pigment Carboxylic Carboxylic Carboxylic Carboxylic CarboxylicCarboxylic Carboxylic groups groups groups groups groups groups groupsCharacteristics Discharge Excellent Excellent Excellent ExcellentExcellent Excellent Good stability I Drying property Excellent ExcellentExcellent Excellent Excellent Excellent Excellent Print sharpness IExcellent Excellent Excellent Excellent Excellent Excellent Excellent

TABLE 2 Example 8 9 10 11 12 13 14 Polar aprotic Molecular weight 86.1114.2 114.2 114.2 114.2 114.2 114.2 solvent P Content 48.5 40 70 75 48.548.5 48 proportion (wt %) Dynamic contact angle of the 16.1° 16.1° 16.1°16.1° 19.8° 24.3° 16.1° surfactant S P/S 61/1 57/1 64/1 63/1 61/1 61/140/1 Modification of the pigment Carboxylic Carboxylic CarboxylicCarboxylic Carboxylic Carboxylic Carboxylic groups groups groups groupsgroups groups groups Characteristics Discharge Excellent ExcellentExcellent Good Excellent Excellent Good stability I Drying propertyExcellent Good Excellent Excellent Excellent Excellent Excellent Printsharpness I Excellent Good Excellent Excellent Excellent Good Good

TABLE 3 Example Comparative Example 15 16 17 18 1 2 3 Polar aproticMolecular weight 114.2 114.2 114.2 114.2 114.2 114.2 157.3 solvent PContent 49.5 48.5 52.5 48.5 35 77.9 48.5 proportion (wt %) Dynamiccontact angle of the 16.1° 16.1° 16.1° 16.1° 16.1° 16.1° 16.1°surfactant S P/S 45/1 69/1 75/1 61/1 61/1 66/1 61/1 Modification of thepigment Carboxylic Carboxylic Carboxylic Sulfonic Carboxylic CarboxylicCarboxylic groups groups groups groups groups groups groupsCharacteristics Discharge Excellent Excellent Excellent Good ExcellentPoor Poor stability I Drying property Excellent Excellent ExcellentExcellent Poor Excellent Excellent Print sharpness I Excellent ExcellentGood Excellent Poor Excellent Excellent

The results for Examples 1 to 8 and Comparative Example 3 in the Tablesshow that a polar aprotic solvent having a molecular weight to 40 to 130must be used as the polar aprotic solvent, and the results for Examples1 and 9 to 11 and Comparative Examples 1 and 2 show that the contentproportion of the polar aprotic solvent must be 40 to 75 weight %. Theresults for Examples 1, 12, and 13 show that the dynamic contact angleat 23° C. of the surfactant is preferably no more than 20°, the resultsfor Examples 1 and 14 to 17 show that the blending ratio (weight ratio)P/S of the polar aprotic solvent P and the silicone-based surfactant Sis preferably in the range of 45/1 to 70/1, and the results of Examples1 and 18 show that the surface of the pigment is preferably modified bycarboxylic groups.

Example 19

<Manufacture of the Inkjet Ink>

A carbon black dispersion [Cabojet 300, manufactured by CabotCorporation, carboxylic group modified, aqueous dispersion, 15 weight %of solids] was used as the pigment, and 1,3-dimethyl-2-imidazolidinone[molecular weight: 114.2, boiling point: 225.5° C.] was used as thepolar aprotic solvent. As the surfactants, SILWET L-77, manufactured byDow Corning Toray Silicone Co., Ltd., was used as the silicone-basedsurfactant expressed by the Formula (1), and SILWET L-720, manufacturedby Dow Corning Toray Silicone Co., Ltd., was used as the silicone-basedsurfactant expressed by the Formula (2).

An ethylene oxide adduct of sodium citrate, with which the p in theFormula (4) is 6 and all of M¹ to M³ are sodium, was used as theethylene oxide adduct of organic acid. As the binder resin, analkali-soluble acrylic resin [NeoCryl B-817, manufactured by AveciaLimited, weight average molecular weight Mw: 23,000] was used, and asthe basic substance for dissolving the binder resin in the inkjet ink,2-amino-2-methyl-1-propanol was used. As the polyoxyethylene phenylether, the compound expressed by the Formula (5-1) was used, and as thepolyoxyethylene alkyl phenyl ether, the compound expressed by theFormula (6-1) was used. As the acetylene glycols, Surfynol 420 [made upof a mixture of a plurality of components of the Formula (7) that differin the values of s and t, with the average value of s+t being 1.3],manufactured by Air Products and Chemicals Inc., was used, and as thebiocidal agent, Proxel XL-2, manufactured by Zeneca Co., Ltd. was used.

The respective components mentioned above were blended at theproportions given below in ion-exchanged water, and after mixing bystirring, the mixture was filtered using a membrane filter of 5 μm tomanufacture the inkjet ink. The blending ratio (weight ratio) S₁/S₂ ofthe two types of the silicone-based surfactants S₁ and S₂ was 5/5, andthe blending ratio (weight ratio) P/(S₁+S₂) of the polar aprotic solventP and the silicone-based surfactants S₁ and S₂ was 48.5/1. (Component)(Parts by weight) Carbon black dispersion 151,3-dimethyl-2-imidazolidinone 48.5 SILWET L-77 0.5 SILWET L-720 0.5Binder resin 1 2-amino-2-methyl-1-propanol 0.1 Ethylene oxide adduct ofsodium citrate 0.5 C₆H₅O(CH₂CH₂O)₆H 0.5 C₈H₁₇C₆H₄O(CH₂CH₂O)₂₅H 0.5Ethanol 2 Acetylene glycols 1 Biocidal agent 0.2 Ion-exchanged water29.7

Examples 20 to 23

Except setting the blending ratio (weight ratio) S₁/S₂ of SILWET L-77,manufactured by Dow Corning Toray Silicone Co., Ltd. as thesilicone-based surfactant expressed by the Formula (1), and SILWETL-720, manufactured by Dow Corning Toray Silicone Co., Ltd. as thesilicone-based surfactant expressed by the Formula (2) so that S₁/S₂=4/6(Example 20), S₁/S₂=6/4 (Example 21), S₁/S₂=3/7 (Example 22), andS₁/S₂=7/3 (Example 23), the total blending amount of the twosilicone-based surfactants S₁ and S₂ to 1 part by weight, and theblending ratio (weight ratio) P/(S₁+S₂) of the polar aprotic solvent Pand the silicone-based surfactants S₁ and S₂ to 48.5/1, inkjet inks weremanufactured in the same manner as in Example 19.

The above-described discharge stability test I and drying test, and thefollowing tests were carried out on the inkjet inks manufactured in therespective Examples to evaluate the characteristics of the inks.

<<pH Measurement>>

The pH of the inkjet ink of each Example was measured using HM-40V,manufactured by DKK-Toa Corporation.

<<Print Sharpness Test II>>

Using the same thermal jet type inkjet ink printer [Desk Jet 970Cxi,manufactured by Hewlett-Packard Development Company, L.P.] as describedabove, printing alphabetical characters of 10-point by each of theinkjet inks of the Examples was performed on each of a glossy offsetcoated medium with a 60° glossiness, as measured, by a micro-Tri-gloss,manufactured by BYK-Gardner GmbH, of 85.8° and a matted offset coatedmedium with a 60° glossiness, as measured in the same manner, of 17.2°,and then after heating for 2 seconds using an oven toaster at an outputof 500 W, the prints were observed and the print sharpness of the inkjetink was evaluated according to the following standards.

Excellent: Edges were expressed sharply. The print sharpness wasevaluated extremely good.

Good: Although there were some portions that somewhat lack sharpness,the print sharpness was evaluated as reaching a practical level.

Poor: The print is not sharp, and the print sharpness was evaluatedpoor. The results of the above are shown in Table 4. TABLE 4 Example 1920 21 22 23 Polar Molecular weight 114.2 114.2 114.2 114.2 114.2 aproticContent 48.5 48.5 48.5 48.5 48.5 solvent P proportion (wt %)Silicone-based surfactants   5/5   4/6   6/4   3/7   7/3 S₁/S₂ P/(S₁ +S₂) 48.5/1 48.5/1 48.5/1 48.5/1 48.5/1 pH 7.40 7.42 7.58 7.55 7.52Characteristics Discharge Excellent Excellent Excellent ExcellentExcellent stability I Drying property Excellent Excellent ExcellentExcellent Excellent Print Glossy Excellent Excellent Excellent GoodExcellent sharpness Matted Excellent Excellent Excellent Excellent GoodII

The results for Examples 19 to 23 in the Table show that when asilicone-based surfactant expressed by the Formula (1) and asilicone-based surfactant expressed by the Formula (2) are used incombination as the surfactants, good printing can be performed withstability on offset coated media of various surface states of a widerange from glossy to matted. Comparison of the Examples shows that theblending ratio (weight ratio) S₁/S₂ of the two silicone-basedsurfactants S₁ and S₂ is preferably S₁/S₂=6/4 to 4/6.

Example 24

A carbon black dispersion [Cabojet 300, manufactured by CabotCorporation, carboxylic group modified, aqueous dispersion, 15 weight %of solids] was used as the pigment, and 1,3-dimethyl-2-imidazolidinone[molecular weight: 114.2, boiling point: 225.5° C.] was used as thepolar aprotic solvent. As the surfactant, Surflon S-111N, manufacturedby Seimi Chemical Co., Ltd. as a fluorine-based surfactant expressed bythe Formula (3) was used.

An ethylene oxide adduct of sodium citrate with which the p in theFormula (4) is 6 and all of M¹ to M³ are sodium was used as the ethyleneoxide adduct of organic acid. As the binder resin, an alkali-solubleacrylic resin [NeoCryl B-817, manufactured by Avecia Limited, weightaverage molecular weight Mw: 23,000] was used, and as the basicsubstance for dissolving the binder resin in the inkjet ink,2-amino-2-methyl-1-propanol was used. As the polyoxyethylene phenylether, the compound expressed by the Formula (5-1) was used, and as thepolyoxyethylene alkyl phenyl ether, the compound expressed by theFormula (6-1) was used. As the acetylene glycols, Surfynol 420 [made upof a mixture of a plurality of components of the Formula (7) that differin the values of s and t, with the average value of s+t being 1.3],manufactured by Air Products and Chemicals Inc., was used, and as thebiocidal agent, Proxel XL-2, manufactured by Zeneca Co., Ltd. was used.

The respective components mentioned above were blended at theproportions given below in ion-exchanged water, and after mixing bystirring, the mixture was filtered using a membrane filter of 5 μm tomanufacture the inkjet ink. The blending ratio (weight ratio) P/F of thepolar aprotic solvent P and the fluorine-based surfactant (effectivecomponent in Surflon S-111N) F was 67.4/1. (Component) (Parts by weight)Carbon black dispersion 15 1,3-dimethyl-2-imidazolidinone 48.5 S-111N2.4 Binder resin 1 2-amino-2-methyl-1-propanol 0.1 Ethylene oxide adductof sodium citrate 0.5 C₆H₅O(CH₂CH₂O)₆H 0.5 C₈H₁₇C₆H₄O(CH₂CH₂O)₂₅H 0.5Ethanol 2 Acetylene glycols 1 Biocidal agent 0.2 Ion-exchanged water28.3

Example 25

Except setting the amount of Surflon S-111N as the fluorine-basedsurfactant to 1.7 parts by weight, the blending ratio (weight ratio) P/Fof the polar aprotic solvent P and the fluorine-based surfactant F to95.1/1, and the amount of the ion-exchanged water to 29.0 parts byweight, an inkjet ink was manufactured in the same manner as in Example24.

Example 26

Except setting the amount of Surflon S-111N as the fluorine-basedsurfactant to 1.5 parts by weight, the blending ratio (weight ratio) P/Fof the polar aprotic solvent P and the fluorine-based surfactant F to107.8/1, and the amount of the ion-exchanged water to 29.2 parts byweight, an inkjet ink was manufactured in the same manner as in Example24.

Example 27

Except setting the amount of Surflon S-111N as the fluorine-basedsurfactant to 3.5 parts by weight, the blending ratio (weight ratio) P/Fof the polar aprotic solvent P and the fluorine-based surfactant F to46.2/1, and the amount of the ion-exchanged water to 27.2 parts byweight, an inkjet ink was manufactured in the same manner as in Example24.

Example 28

Except setting the amount of Surflon S-111N as the fluorine-basedsurfactant to 3.8 parts by weight, the blending ratio (weight ratio) P/Fof the polar aprotic solvent P and the fluorine-based surfactant F to42.5/1, and the amount of the ion-exchanged water to 26.9 parts byweight, an inkjet ink was manufactured in the same manner as in Example24.

The above-described discharge stability test I, drying test, and printsharpness test II were carried out on the inkjet inks manufactured inthe respective Examples to evaluate the characteristics of the inks. Theresults are shown in Table 5. TABLE 5 Example 24 25 26 27 28 PolarMolecular weight 114.2 114.2 114.2 114.2 114.2 aprotic Content 48.5 48.548.5 48.5 48.5 solvent P proportion (wt %) P/F 67.4/1 95.1/1 107.8/146.2/1 42.5/1 Characteristics Discharge Excellent Excellent ExcellentExcellent Good stability I Drying property Excellent Excellent ExcellentExcellent Excellent Print Glossy Excellent Excellent Good ExcellentExcellent sharpness Matted Excellent Excellent Excellent ExcellentExcellent II

The results for Examples 24 to 28 in the Table show that when afluorine-based surfactant expressed by the Formula (3) is used as thesurfactant, good printing can be performed with stability on offsetcoated media of various surface states of a wide range from glossy tomatted. Comparison of the Examples shows that the blending ratio (weightratio) P/F of the polar aprotic solvent P and the fluorine-basedsurfactant F is preferably such that P/F=45/1 to 100/1.

Example 29

Except using the same amount of Johncryl 682 (weight average molecularweight Mw: 1,700), manufactured by Johnson Polymer Corporation, as thebinder resin, an inkjet ink was manufactured in the same manner as inExample 24.

Example 30

Except using the same amount of Johncryl 586 (weight average molecularweight Mw: 4,600), manufactured by Johnson Polymer Corporation, as thebinder resin, an inkjet ink was manufactured in the same manner as inExample 24.

The above-described discharge stability test I, drying test, printsharpness test II, and the following test were carried out on the inkjetinks manufactured in the respective Examples to evaluate thecharacteristics of the inks.

<Discharge Stability Test II>>

Each of the inkjet inks of the Examples and Comparative Examples wasfilled into an empty black ink cartridge [Part No. 51645A, manufacturedby Hewlett-Packard Development Company, L.P.] for a thermal jet typeinkjet ink printer [Desk Jet 6127, manufactured by Hewlett-PackardDevelopment Company, L.P.], and printing was performed. Then afterleaving the printer in a state in which the head is not returned to thehome position and the nozzles are uncapped for 5 minutes, solid printingof 1 cm×1 cm was performed on a commercially available glossy paper. Theprint was then observed, and the discharge stability of the inkjet inkwas evaluated according to the following standards.

Excellent: No faints were seen at all in the solid print. The dischargestability after restarting of printing was evaluated extremely good.

Good: Although faints were seen slightly at initial portions of thesolid print, solid printing without faints was performed thereafter. Thedischarge stability after restarting of printing was evaluated asreaching a practical level.

Poor: Faints were seen across the entire solid print. The dischargestability after restarting of printing was evaluated poor.

The results of the above are shown in Table 6. TABLE 6 Example 29 30Polar aprotic Molecular weight 114.2 114.2 solvent P Content proportion48.5 48.5 (wt %) P/F 67.4/1 67.4/1 Molecular weight Mw of the binderresin 1,700 4,600 Characteristics Discharge stability I ExcellentExcellent Discharge stability II Excellent Good Drying propertyExcellent Excellent Print Glossy Excellent Excellent sharpness II MattedExcellent Excellent

The results for Examples 29 and 30 in the Table show that, in an inkjetink used in an inkjet printer of a type with which the operation ofreturning the head to the home position is performed manually, the useof a binder resin with a weight average molecular weight Mw of 1,000 to3,000 is preferable for preventing faints and other print defects.

1. An inkjet ink for printing on hydrophobic media comprising: apigment; water; a surfactant; and a polar aprotic solvent having amolecular weight of 40 to 130, wherein the content proportion of thepolar aprotic solvent is 40 to 75 weight %.
 2. The inkjet ink accordingto claim 1, wherein a boiling point of the polar aprotic solvent is 150to 250° C.
 3. The inkjet ink according to claim 1, wherein the polaraprotic solvent is at least one type of solvent selected from the groupconsisting of 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone,2-pyrrolidone, formamide, N-methylformamide, N,N-dimethylformamide,N,N-diethylpropionamide, and γ-butyrolactone.
 4. The inkjet inkaccording to claim 1, wherein the surfactant is a surfactant having adynamic contact angle at 23° C. of no more than 20°.
 5. The inkjet inkaccording to claim 4, wherein the surfactant is a silicone-basedsurfactant, and a blending ratio (weight ratio) P/S of the polar aproticsolvent P and the silicone-based surfactant S is P/S=45/1 to 70/1.
 6. Aninkjet ink comprising: a pigment; water; a surfactant; and polar aproticsolvent having a molecular weight of 40 to 130, wherein the contentproportion of the polar aprotic solvent is 40 to 75 weight %, as thesurfactant, a silicone-based surfactant S₁ expressed by the Formula (1):[Chemical Formula 1]

[where R¹ indicates a polyalkylene oxide chain containing at least oneor more ethylene oxide groups or propylene oxide groups, and x indicatesa numeral of no less than 1] and a silicone-based surfactant S₂expressed by the Formula (2): [Chemical Formula 2]

[where R² indicates a polyalkylene oxide chain containing at least oneor more ethylene oxide groups or propylene oxide groups, and y indicatesa numeral of no less than 1] are used in combination, and a blendingratio (weight ratio) S₁/S₂ of the two silicone-based surfactants isS₁/S₂=614 to 4/6.
 7. The inkjet ink according to claim 6, wherein ablending ratio (weight ratio) P/(S₁+S₂) of the polar aprotic solvent Pand the two silicone-based surfactants S₁+S₂ is P/(S₁+S₂)=45/1 to 70/1.8. An inkjet ink for printing on hydrophobic media comprising: apigment; water; a surfactant; and polar aprotic solvent having amolecular weight of 40 to 130, wherein the content proportion of thepolar aprotic solvent is 40 to 75 weight %, and the surfactant is afluorine-based surfactant expressed by the Formula (3): [ChemicalFormula 3]C_(m)F_(2m+1)COONH₄   (3) [where m indicates a numeral of no less than1].
 9. The inkjet ink according to claim 8, wherein a blending ratio(weight ratio) P/F of the polar aprotic solvent P and the fluorine-basedsurfactant F is P/F=45/1 to 100/1.
 10. The inkjet ink according to claim1, wherein the pigment surface is modified by carboxylic groups.
 11. Aprinting method comprising the steps of: using the inkjet ink of claim10 to print on a hydrophobic medium; and heating the hydrophobic mediumafter printing.
 12. A printing method comprising the steps of: using theinkjet ink of claim 9 to print on a hydrophobic medium; and heating thehydrophobic medium after printing.
 13. A printing method comprising thesteps of: using the inkjet ink of claim 8 to print on a hydrophobicmedium; and heating the hydrophobic medium after printing.
 14. Aprinting method comprising the steps of: using the inkjet ink of claim 7to print on a hydrophobic medium; and heating the hydrophobic mediumafter printing.
 15. A printing method comprising the steps of: using theinkjet ink of claim 6 to print on a hydrophobic medium; and heating thehydrophobic medium after printing.
 16. A printing method comprising thesteps of: using the inkjet ink of claim 5 to print on a hydrophobicmedium; and heating the hydrophobic medium after printing.
 17. Aprinting method comprising the steps of: using the inkjet ink of claim 4to print on a hydrophobic medium; and heating the hydrophobic mediumafter printing.
 18. A printing method comprising the steps of: using theinkjet ink of claim 3 to print on a hydrophobic medium; and heating thehydrophobic medium after printing.
 19. A printing method comprising thesteps of: using the inkjet ink of claim 2 to print on a hydrophobicmedium; and heating the hydrophobic medium after printing.
 20. Aprinting method comprising the steps of: using the inkjet ink of claim 1to print on a hydrophobic medium; and heating the hydrophobic mediumafter printing.